Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a substrate or wafer. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. When inspecting specular or quasi-specular surfaces, such as semiconductor wafers, any combination of bright field (BF) and dark field (DF) modalities may be used, both to perform patterned wafer inspection and defect review. In general, the defect sensitivity of an inspection system is proportional to the wavelength of the illumination light divided by the NA of the objective. Without further improvement in NA, the overall defect sensitivity of current inspection tools is limited by the wavelength of the illumination source. In addition, viable inspection systems require high radiance illumination to maximize the defect sensitivity of the system.
Current wafer inspection systems typically employ illumination sources of deep ultraviolet (DUV) radiation with wavelengths as short as 260 nanometers with a high numerical aperture (NA). In some examples, illumination light may provided by an arc lamp. For example, electrode based, relatively high intensity discharge arc lamps are used in inspection systems. However, these light sources have a number of disadvantages. For example, electrode based, relatively high intensity discharge arc lamps have radiance limits and power limits due to electrostatic constraints on current density from the electrodes, the limited emissivity of gases as black body emitters, the relatively rapid erosion of electrodes made from refractory materials due to the presence of relatively large current densities at the cathodes, and the inability to control dopants (which can lower the operating temperature of the refractory cathodes) for relatively long periods of time at the required emission current.
In some other examples, illumination light is provided by a laser. One approach to realize the desired short wavelength emission has been the harmonic upconversion of longer wavelength sources. However, the average power that can be reliably sustained has been limited. A significant reliability problem is the failure of nonlinear crystals used in harmonic generators operating at relatively high average power.
Shorter wavelength illumination sources with the required radiance, average power, and reliability for future semiconductor processing applications are required. In particular, improvements in the reliability of crystal elements of relatively high average power lasers employing harmonic upconversion are desired.